When a magneto-optical recording method is adopted, a recording medium is employed which includes a substrate having thereon a perpendicular magnetic film made of a magnetic substance, and recording and reproducing operations on and from the recording medium are performed in the following way.
When a recording operation is to be carried out, first, the magnetization direction of the recording medium is arranged in one direction (upward or downward) by applying thereto a strong external magnetic field so as to initialize the recording medium. Then, a laser beam is projected onto a recording area of the recording medium so as to raise a temperature thereof above the vicinity of its Curie temperature or above the vicinity of its compensation temperature. As a result, a coercive force (Hc) at the heated portion becomes zero or substantially zero. Further, an external magnetic field (bias magnetic field) having an opposite direction to an initialization magnetic direction is applied, thereby reversing the magnetization direction. Then, the projection of the laser beam is stopped. As the temperature of the recording medium drops to a room temperature, the reversed magnetization direction is fixed, thereby recording information thermomagnetically.
When a reproducing operation is to be carried out, a linearly polarized laser beam is projected onto the recording medium, and the recorded information is optically read out utilizing an effect that the polarization plane of a reflected light or transmitted light rotates differently depending on the magnetization direction (magnetic Kerr effect, or magnetic Faraday effect).
The magneto-optical recording medium designed for the magneto-optical recording method has been viewed with interest as a rewritable high density and large capacity memory device. In order to rewrite on the magneto-optical recording medium, either one of the following methods (a), (b), and (c) is required.
(a) initializing through any method; PA1 (b) devising an external magnetic field (bias magnetic field) generation device so as to enable the overwriting operation (rewriting operation without requiring an erasing operation); and PA1 (c) devising the recording medium so as to enable the overwriting operation (rewriting operation without requiring an erasing operation).
However, when the method (a) is adopted, either an initialization device is required, or two magnetic heads are required, thereby presenting the problem of high cost. Or when erasing using only one magnetic head, there arises the problem that the erasing operation requires the same amount of time required for recording. On the other hand, when the method (b) is adopted, the magnetic head may be crushed as in the case of the magnetic recording.
Thus, the method (c) of devising the recording medium is the most effective method. As for this method, by employing a double-layered exchange coupled film for a recording layer, the overwriting operation is enabled as reported by Akasaka et al (for example, see Jap. Jour. Appl. Phys., Vol. 28 (1989) Suppl. 28-3, pp. 367-370).
The processes for the overwriting operation will be briefly described below. As shown in FIG. 16, in the magneto-optical recording medium composed of a first magnetic layer 56 and a second magnetic layer 57, an initialization magnetic field H.sub.ini is applied thereto so as to arrange the magnetization in the second magnetic layer 57 in one direction (downward in the figure) in order to initialize the recording medium. Here, the initialization may always be performed or performed only when recording. In this state, since a coercive force H.sub.1 of the first magnetic layer 56 is greater than the initialization magnetic field H.sub.ini, the magnetization in the first magnetic layer 56 needs not be reversed as shown in FIG. 18.
A recording operation is performed by projecting a laser light which is to be switched between a high level and a low level, while applying a recording magnetic field H.sub.w. In more detail, a laser beam of the high level denoted by I in FIG. 17 is set so as to raise both the temperatures of the first magnetic layer 56 and the second magnetic layer 57 to the temperature T.sub.H which is in the vicinity of the Curie temperature T.sub.2 or above the Curie temperature T.sub.2. On the other hand, a laser beam of the low level denoted by II in FIG. 17 is set so as to raise only the temperature of the first magnetic layer 56 to the temperature T.sub.L which is in the vicinity of its Curie temperature T.sub.1 or above the Curie temperature T.sub.1.
Therefore, when projecting the laser beam of the high level I, the magnetization in the second magnetic layer 57 is reversed (upward in FIG. 16) by applying thereto the recording magnetic field H.sub.w. Then, the magnetization in the first magnetic layer 56 is also reversed in the magnetization direction of the second magnetic layer 57 using an exchange force exerted on an interface in the process of cooling off.
On the other hand, when projecting the laser beam of the low level II, the magnetization direction in the second magnetic layer 57 is not reversed by the recording magnetic field H.sub.w. In this case also, the magnetization direction in the first magnetic layer 56 is arranged in the magnetization direction of the second magnetic layer 57 in the process of cooling off. Thus, the magnetization direction in the first magnetic layer 56 shows downward in FIG. 16. Additionally, the recording magnetic field H.sub.w is set significantly smaller than the initialization magnetic field H.sub.ini as shown in FIG. 18. Furthermore, as denoted by III of FIG. 17, the intensity of the laser beam used in reproducing is set significantly lower than the laser beam used in recording.
However, when the above method is adopted, a large initialization magnetic field is required. Or if a double-layered film which makes the initialization magnetic field smaller is employed, a stable recording operation cannot be preformed, and therefore it is not possible to perform an overwriting operation. In order to counteract this, a three-layered recording layer may be adopted having an intermediate layer between two exchange coupled films (for example, see Jap. Jour. Appl. Phys., Vol. 28(1989) Suppl. 28-3, pp. 27-31). However, the three-layered recording layer presents the problems of an increase in the manufacturing cost and complicated manufacturing process.